"Pot bound"

Aside from conserving space, keeping plants in cramped quarters is a hedge
against overwatering. If you overpot and are very frugal in watering it may
not be a problem but most of us have a tendency to overwater. Smaller bulbs
that grow in rocky terrain or crevices naturally are suited to pot bound
conditions. Ideally, bulbs should not be given a larger container than they
actually need-- most plants grow better when their roots can reach the
sides of the pot (see below). Frequent repotting-- preferably when the
plant is active-- is better than one sudden move to a pot that is a lot
larger than the bulb. Usually underpotting is a tolerable constraint;
overpotting is much more likely to be lethal.
I've noticed that in pots there is a "sweet spot" where a plant is most
ideally suited to its situation. This is not right after transplanting,
when growth may be slow, nor years later when the roots are really
congested. Rather it is when the roots have filled out the pot and can
obtain an air:///water/ ratio that they enjoy, with the roots mostly at the
sides and bottom of the container. Roots tend to migrate to these areas of
the pot to seek air, water movement and warmth. This seems to be universal
behavior in containerized plants. Near the center of the container volume
these factors are not as ideal, and the larger the pot, the larger will be
this central zone with colder, wetter and less aerated conditions. On the
other hand, cold climate species may benefit from just such a refuge from
heat and drying.
There is another "sweet spot" in between watering and drying. When first
watered the soil has a high water: air ratio and the plant is not very
active. Only after the soil has dried somewhat, allowing air to replace
some of the water in the spaces between soil particles, are conditions
ideal for growth below the soil (temperature, root hair absorption, etc.)
and above ground. This is often visible over one or two days in plants
under ideal conditions.
Dylan Hannon
*"The greatest service which can be rendered any country is to add an
useful plant to its culture..." --**Thomas Jefferson*